It's a tricky level - you need a bit of luck that the AI chases you to the west side, while you do some clever manoeuvring to keep/retake as many cities as you can.
One thing you can do that makes this notably easier, is that if you get the right luck at the start you can have 2 cities at the start of Turn 5 - I uploaded a screenshot at the link below:https://ibb.co/nwDn10w
You can get this ~25% of the time - you need the AI to hit your city first Turn 3, so you can run, and then you need the AI to hit your 1 in the north and not hit south instead (and you make add an army to that 1 and attack killing off his armies there). Both of these are 50:50 coin flips. So, I would suggest you keep restarting until you get this position (or something similar) at start of Turn 5, as that extra city really helps.